1. Field of the Invention
The present invention relates generally to fabrication of solid state electron devices by methods of thick film deposition and subsequent processing, which provides versatility and economic high volume production. The methods are applied to thermoelectric devices to provide advantages towards ultimately lowering the cost per watt for energy harvesting or thermoelectric coolers.
2. Description of the Related Art
Walton discusses the general aspects of thick films and their formulation. For the formulation of thick film pastes, powdered material of metal and/or glass are mixed until homogenous with a vehicle. When the vehicle and solid state powders are combined in the appropriate volume ratios, a thixotropic paste results which can be screen or stencil printed onto substrates. For some specific applications, the vehicle is a polymer with a molecular weight that yields the desired thixotropic properties required for printing when mixed with a powder that is chemically inert with the polymer. In many cases, the vehicle is a polymer can be poly-methyl methacrylate, ethyl cellulose, or more recently, methyl styrene. Methyl styrene is a polymer that completely burns off without residue at 350° C. This complete volatilization is important in the fabrication of thermoelectric thick films, where purity is of importance for electrical and thermal properties, as is well-known in the field of thermoelectric devices, especially for power generation. Conventional thick film structures for conductors and resistors are generally printed on electrically insulating substrates such as alumina or other ceramic materials, and electrical conduction is parallel to the substrate. In the case of thermoelectric devices, at least in this invention, electrical conducting substrates are used, and conduction is vertical to the substrate. Thus, the two properties of cohesion within the thick film and adhesion to the substrate are important, as cohesion determines the electrical and thermal performance critical to device performance, and adhesion is related to contact resistance, which can directly affect conversion efficiency.
Research in thick film methods have been reported, though there has been no progress in developing a process by which the semiconductor remains pure or is directly bonded to the metal substrate. Ohta et al [3] first published work on thermoelectric thick films of the same kind as in this invention, but a glass frit of lead oxide was usually added and the films were not densified by pressure, nor are there any claims in the patents issued to Ohta or Tokiai similar as reported in this invention. Markowski et al used metal inks, and not the conventional doped semiconductors as are common today.
Xi et al used an epoxy resin as a binder, while Navone et al used a 2% by weight polystyrene. In Navone's work, even though a densification process was reported, the polystyrene wasn't volatilized during the heat treatment at 350° C., as this temperature is well below the decomposition temperature. This invention allows heat treatment without cracking and delamination of screen printed films directly onto metal substrates because of the high free volume. In addition, no metal was in contact with the semiconductor during heat treatment in Navone's work.
Lee et al in two publications used a glass binder for ZnSb and CoSb3 and densified the films by high temperature processing which resulted in porous films. Madan et al used an 2% by weight epoxy binder with the thermoelectric materials that remained in the thick films. Kim et al removed the organic binder and used hot pressing for densification of the thick films on alumina substrates in a hydrogen atmosphere, hence the structures were not metallized during this high temperature processing. We et al printed Bi2Te3 powders mixed with a glass binder onto oxidized silicon wafers and densified by sintering without applying pressure, resulting in relatively porous films.
No one has reported a direct bond process for fabricating densified thermoelectric semiconductor thick films on metal substrates as provided in this invention. There is a need for such a process in order to lead to high volume manufacturing of thermoelectric device applications at lower cost.